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Single crystals of Ir2S3 (diiridium trisulfide) and Rh2S3 (dirhodium trisulfide) were grown in evacuated silica‐glass tubes using a chemical transport method and their crystal structures were determined by single‐crystal X‐ray diffraction analysis. These compounds have a unique sesquisulfide structure in which pairs of face‐sharing octahedra are linked into a three‐dimensional structure by further edge‐ and vertex‐sharing. Ir2S3 and Rh2S3 had similar unit‐cell parameters and bond distances. The atomic displacement parameter (MSD: mean‐square displacement) of each atom in Ir2S3 was considerably smaller than that in Rh2S3. The Debye temperatures (ΘD) estimated from the observed MSDs for the Ir, S1 and S2 sites in Ir2S3 were 259, 576 and 546 K, respectively, and those for Rh, S1 and S2 in Rh2S3 were 337, 533 and 530 K, respectively. The bulk Debye temperature for Ir2S3 kashinite (576 K) was found to rank among the higher values reported for many known sulfides. The bulk Debye temperature for Rh2S3 bowieite (533 K) was lower than that for Ir2S3 kashinite, which crystallizes in the early sequences of mineral crystallization differentiation from the primitive magma in the Earth's mantle. [ABSTRACT FROM AUTHOR]

Single crystals of CaIr[O.sub.3] were prepared via flux growth method. Crystal structure parameters, including the anisotropic displacement parameters, are determined based on a single-crystal X-ray diffraction experiment. The unit-cell dimensions are a = 3.147(2), b : 9.866(6), and c = 7.302(5) [Angstrom]. The structure is a three-dimensional dense structure with small vacant spaces. The CaIr[O.sub.3] structure can be described as a pseudo-one-dimensional oxide and is compared with [Ca.sub.4]Ir[O.sub.6] structure. The Ir[O.sub.6] octahedra are significantly distorted, in contrast to other octahedral [Ir.sup.4+] compounds. The O-O distances for faces and edges shared between polyhedra are shorter than other non-shared edge distances. These effects are explained by Pauling's rules and occur to decrease the repulsion between the cations. Thermal vibrations of Ca and Ir atoms are significantly anisotropic. Thermal vibrations of Ca and Ir atoms are restricted in orientation toward the shared face, shared edges, and shortest cation-cation directions. The single-crystal experiment shows that CaIr[[O.sub.3] crystals grow fastest along the a axis and that they assume a prism or needle shape. Strongly preferred orientation of such prism shaped CaIr[O.sub.3]-type post perovskite MgSi[O.sub.3] crystals may develop under the shear flow in the Earth's mantle. Keywords: Post-perovskite, CaIr[O.sub.3], single crystal, X-ray diffraction, lower mantle, D' layer

High-pressure single-crystal X-ray diffraction experiments of MgSi[O.sub.3] perovskite have been carried out up to 15 GPa in a diamond-anvil cell using synchrotron radiation. Precise crystal structural parameters, including the anisotropic displacement parameters of every atom in MgSi[O.sub.3], are determined under high pressure. In the pressure range up to 15 GPa, the most important responses of the structure are the compressions of Si[O.sub.6] and Mg[O.sub.8] polyhedra and an increase in tilting of Si[O.sub.6] octahedra represented by the decrease in angles between octahedra (both Si-O2-Si angle in the a-b plane and Si-O1-Si angle in the b-c plane decrease). The degree of the change in both angles in the a-b and b-c planes is the same. The amplitude of mean square displacement for the Mg atom has the largest value in the structures and its thermal vibration is significantly anisotropic at ambient pressure. Under high pressure, all atoms in the structure have obvious anisotropy of thermal vibration and the largest amplitudes of thermal vibration for Mg, Si, and O2 atoms are directed toward vacant space in the structure. Anisotropy of the structure increases with pressure. Keywords: Crystal structure, high-pressure study, MgSi[O.sub.3] perovskite, pressure responses, thermal vibration

Six single crystals of [Mg.sub.3]([Mg.sub.x][Si.sub.x][Al.sub.2-2x][Si.sub.3][O.sub.12] with x = 0.05, 0.13, 0.24, 0.38, 0.52, and 0.64 (the majorite solid-solution) were synthesized at 20 GPa and 2000 [degrees] C with a '6-8' type uniaxial split-sphere apparatus. Single-crystal X-ray diffraction studies revealed discontinuities in compositional dependence of the molar volume, equivalent isotropic temperature factors ([B.sub.eq]), and mean bond lengths between x = 0.24 and 0.38. Single crystals in the compositional range 0 [less than or equal to] x [less than or equal to] 0.24 show no birefringence, whereas those of x = 0.64 have a slight optical anisotropy. Moreover, the cell symmetry for x = 0.64 obtained using synchrotron X-ray radiation is tetragonal with a slight deviation from cubic. On the basis of site splitting expected from compositional dependence of [B.sub.eq] obtained by cubic refinement, the most probable space group in the range 0.38 [less than or equal to] x [less than or equal to] 0.64 is I[4.sub.1]/acd (tetragonal), which is the maximal subgroup of the space group [Mathematical Expression Omitted] (cubic). Given that the previous reports that crystals with 0.8 [less than or equal to] x [less than or equal to] 1.0 have the tetragonal space group I[4.sub.1]/a, the majorite solid-solution in this system undergoes the series of symmetry changes, [Mathematical Expression Omitted], with increasing MgSi[O.sub.3] component. The symmetry changes from [Mathematical Expression Omitted] to I[4.sub.1]/acd cannot be explained by the cation ordering on the octahedral site. Strong electrostatic interaction between the dodecahedral ([Mg.sup.2+]) and tetrahedral ([Si.sup.4+]) cations was observed from atomic thermal motion and electron density distribution. Because one of the site symmetries of the two nonequivalent tetrahedral sites in I[4.sub.1]/acd structure loses the center of symmetry with the symmetry reduction from [Mathematical Expression Omitted] to I[4.sub.1]/acd, the symmetry reduction may be caused by the electronic polarization of the cations due to the neighboring cation-cation interaction.

A single crystal of a birefringent Cr-bearing majorite,Mg3[([Mg.su .0.34][Si.sub.0.34][Al.sub.0.18][Cr.sub.0.14]).sub.2][Si.sub.3][O.sub.12], was synthesized at 20 GPa and 2000 [degrees] C using '6-8' type uniaxial split-sphere apparatus. This garnet is tetragonal with the unit-cell parameters a [approximately equal to] c and deviates slightly from cubic symmetry. The structure refinements using single-crystal X-ray diffraction intensity data were carried out by assuming three space groups (one cubic and two tetragonal) to determine the most probable symmetry. The most probable space group is [I4.sub.1]/a (tetragonal). The Cr ions show a disordered distribution between the two nonequivalent octahedral sites in the [I4.sub.1]/a structure.

Single crystals of CaGe[O.sub.3] garnet were synthesized at 3 GPa and 1000 [degrees]C using a cubic anvil type of high pressure apparatus and the crystal structure was refined from single crystal X-ray diffraction data. This garnet is tetragonal with lattice parameters of a = 12.535(2) A, c = 12.370(2) A, V = 1943.5(5) [A.sup.3] and belongs to space group I[4.sub.1]/a. Two dodecahedral sites are occupied only by Ca with mean CaO bond lengths of 2.480(4) and 2.467(4) A. The Ca and Ge cations are completely ordered at two octahedral sites with mean Ca-O = 2.301(3) A and mean Ge-O = 1.910(3) A. Three tetrahedral sites are occupied only by Ge, and their mean Ge-O bond lengths are 1.753(3), 1.787(4), and 1.764(4) A. Furthermore. the present tetragonal garnet has an unusual feature in that the mean value [2.704(5) A] of the shared edge lengths of Ge[O.sub.6] octahedron is larger than that [2.699(5) A] of the unshared ones, as has also been observed for other tetragonal garnets with I[4.sub.1]/a.

Single crystals of Ni3Se2 (trinickel diselenide) and NiSe (nickel selenide) with stoichiometric chemical compositions were grown in evacuated silica‐glass tubes. The chemical compositions of the single crystals of Ni3Se2 and NiSe were determined by scanning electron microscopy and energy‐dispersive X‐ray spectroscopy (SEM/EDS). The crystal structures of Ni3Se2 [rhombohedral, space group R32, a = 6.02813 (13), c = 7.24883 (16) Å, Z = 3] and NiSe [hexagonal, space group P63/mmc, a = 3.66147 (10), c = 5.35766 (16) Å, Z = 2] were analyzed by single‐crystal X‐ray diffraction and refined to yield R values of 0.020 and 0.018 for 117 and 85 unique reflections, respectively, with Fo > 4σ(Fo). R32 is a Sohncke type of space group where enantiomeric structures can exist; the single‐domain structure obtained by the refinement was confirmed to be correct by a Flack parameter of −0.05 (2). The existence of Ni—Ni bonds was confirmed in both compounds, in addition to the Ni—Se bonds. The value of the atomic displacement parameter (mean‐square displacement) of each atom in NiSe was larger than that in Ni3Se2. The larger amplitude of the atoms in NiSe corresponds to longer Ni—Se and Ni—Ni bond lengths in NiSe than in Ni3Se2. The Debye temperatures, gθD, estimated from observed mean‐square displacements for Ni and Se in Ni3Se2, were 322 and 298 K, respectively, while those for Ni and Se in NiSe were 246 and 241 K, respectively. The existence of large cavities in the structure and the weak bonding force are likely responsible for the brittle and soft nature of the NiSe crystal. [ABSTRACT FROM AUTHOR]

The stability of hauerite (MnS2) as compared to that of pyrite (FeS2) can be explained by the long Mn–S distance and departure from the typical pyrite-type structures. The structural differences of MnX2 compounds (X=S, Se, and Te) are the result of spin configurations that are different than those of other MX2 compounds; however, the arrangement of d-electrons and the size of the ions in MnX2 compounds do not clearly explain why Mn2+ in MnX2 does not exist as a low spin state. To investigate the structural differences of MnX2 compounds, we synthesized single-crystal MnTe2 and MnSe2 and performed single-crsytal X-ray diffraction experiments. The single-crystal X-ray diffraction experiments were conducted on MnTe2 [a=6.9513(1) Å, u-parameter=0.38554(2), space group Pa3̅, Z=4], MnSe2 [a=6.4275(2) Å, u-parameter=0.39358(2)], MnS2 [hauerite; a=6.1013(1) Å, u-parameter=0.40105(4), obtained from Osorezan, Aomori, Japan], and FeS2 [pyrite; a=5.4190(1) Å, u-parameter 0.38484(5), obtained from Kawarakoba, Nagasaki, Japan]. The X-ray intensity datasets of these compounds do not show any evidence of symmetry reduction. In MnS2, the S–S distance is 2.0915(8) Å, which is significantly shorter than that of FeS2 (2.1618(9) Å), and the mean square displacement of S (U11=0.00915(9) Å2) is smaller than that of Mn (U11=0.01137(9) Å2). The thermal vibration characteristics of MnX2 compounds are significantly different than those of FeS2. Based on structural refinement data, we discuss the low spin state of MnX2 compounds and the structural stability of pyrite-type structures. [ABSTRACT FROM AUTHOR]

Pre‐edge peaks in 3d transition‐metal element (Sc, Ti, V, Cr and Mn) K‐edge XANES (X‐ray absorption near‐edge structure) spectra in AO2 (A = Ti and V), A2O3 (A = Sc, Cr and Mn) and AO (A = Mn) are measured at various temperatures. Quantitative comparisons for the XANES spectra were investigated by using absorption intensity invariant point normalization. The energy position of the difference peak (D peak) is obtained from the difference between the low‐ and high‐temperature XANES spectra. There are two kinds of temperature dependence for pre‐edge peak intensity: rutile‐ and anatase‐type. The true temperature dependence of a transition to each orbital is obtained from the difference spectrum. In both anatase and rutile, the pre‐edge peak positions of A2 and A3 are clearly different from the D1‐ and D2‐peak positions. The A1 peak‐top energies in both phases of VO2 differ from the D1 peak‐top energies. The D‐peak energy position determined by the difference spectrum should represent one of the true energies for the transition to an independent orbital. The peak‐top positions for pre‐edge peaks in XANES do not always represent the true energy for independent transitions to orbitals because several orbital transitions overlap with similar energies. This work suggests that deformation vibration (bending mode) is effective in determining the temperature dependence for the D‐peak intensity. [ABSTRACT FROM AUTHOR]

Single crystals of the vanadate garnet Ca2NaCd2V3O12 (dicalcium sodium dicadmium trivanadate) were synthesized using the floating‐zone method and the crystal structure was investigated using single‐crystal X‐ray diffraction. We considered the effectiveness of substitution of the Y‐site cation with reference to previous structural studies of vanadate garnets. The structures of vanadate garnets are subject to geometric constraints similar to those of silicate garnets. These constraints force the tetrahedral–dodecahedral shared edge length in vanadate garnets to become shorter than the unshared dodecahedral edge length, as in ugrandite (uvarovite, grossular and andradite) garnets. However, the vanadate garnet Ca2NaCd2V3O12 exhibits the normal structural feature, similar to pyralspite (pyrope, almandine and spessartine) garnets, namely that the dodecahedral–dodecahedral shared edge length is shorter than the unshared dodecahedral edge length. With increasing ionic radius of the Y‐site cation, the atomic coordinates x, y and z of oxygen adopt values which satisfy Pauling's third rule. [ABSTRACT FROM AUTHOR]